In recent years, attempts have been made to reduce exhaust gases such as CO2 from the viewpoint of global environmental protection. In the automotive industry, measures have been taken to reduce the amount of exhaust gas in such a way that fuel efficiency is increased by automotive weight reduction.
One of techniques for automotive weight reduction is that a steel sheet for use in automobiles is increased in strength and is thereby reduced in thickness. Steel sheets for use in floors need to have corrosion resistance in addition to increased strength and reduced thickness and therefore the application of high-strength hot-dip galvanized steel sheets is being investigated. Since steel sheets are reduced in ductility with an increase in strength, a steel sheet having both high strength and ductility is needed. Floor parts are formed so as to have a complicated shape in many cases and therefore ductility and stretch flangeability are needed.
In order to cope with such requirements, for example, Patent Literature 1 discloses a method for manufacturing a high-strength hot-dip galvanized steel sheet having low yield ratio, an excellent balance between strength and ductility, and excellent stretch flangeability. In the method, in a continuous galvanizing line, bainite transformation is caused in such a way that a steel sheet is cooled at a predetermined rate in a quenching zone after soaking and is maintained at a predetermined temperature and martensite transformation is caused in such a way that the steel sheet is galvanized, is alloyed, and is then quenched, whereby the microstructure of the steel sheet is transformed to a three-phase microstructure consisting of ferrite, bainite, and martensite.
Patent Literature 2 discloses a method for manufacturing a high-strength steel sheet with excellent formability. In this method, retained austenite is produced in the microstructure of the steel sheet in such a way that the average rate of cooling, subsequent to annealing and soaking, from 650° C. to a temperature at which a steel sheet enters a galvanizing bath or to a temperature of 450° C. is specified and the steel sheet is held at a temperature of 300° C. to 450° C. for a predetermined time before or after being galvanized, whereby the high-strength steel sheet is manufactured so as to have an excellent balance between strength and ductility.
Furthermore, Patent Literature 3 discloses a method for manufacturing a high-strength hot-dip galvanized steel sheet having a tensile strength equivalent to a 440-1,500 MPa class and excellent bendability (λ value (λ: hole expansion ratio)) as evaluated by a hole expansion test in which a hole is expanded using a conical punch with an edge angle of 60° until cracks are formed around the hole. In this method, the content of each component is adjusted within an appropriate range, a reheating step is introduced after a galvanizing step, and further, cooling is performed at a predetermined cooling rate subsequently to a recrystallization annealing step and prior to the reheating step, whereby the microstructure of a steel sheet is transformed to tempered martensite.